A start condition of a charging state machine of a hand-held terminal is to detect whether a Charging Voltage (VCHG) is greater than a certain threshold limit value (e.g., 3.3V), when an external voltage is greater than the value, it is to begin to start the charging state machine and begin a charging management, otherwise the charging is stopped.
A start condition of a USB state machine of a hand-held terminal is to detect whether a voltage of a USB Voltage Bus (USB_VBUS) end is greater than a Session value (e.g., 2.0V) in the USB specification, when an external voltage is greater than the value, it is to begin to start a USB enumeration process, otherwise the USB state machine is stopped.
The China's Ministry of Information Industry (MII) issued Technical Requirements and Test Method of Charger and Interface for Mobile Telecommunication Terminal Equipment (YD/T1591-2009) in November, 2006, it is formulated that a USB communication cable and a USB charging cable are an identical cable in a handset, that is, the cable can serve as a charging cable and also can serve as a USB communication cable, thus the VCHG and the USB_VBUS of a charging interface of mobile phone are shorted together. Currently, in the industry, a common practice of a USB start scheme that a charger and USB communication share one interface is described below.
A. The VCHG and the USB_VBUS are shorted together.
The circuit of this method is comparatively simple, but there exists the following disadvantage, a withstand voltage of the USB_VBUS end of hand-held terminal chip is lower, which cannot satisfy overvoltage protection requirements formulated in the Technical Requirements and Test Method of Charger and Interface for Mobile Telecommunication Terminal Equipment. 
B. The VCHG is used to control the USB_VBUS power-on through a hardware circuit.
This scheme can implement the charging overvoltage protection, but there exists an external circuit, which increases costs.